A conventional multicarrier transfer system is explained below. Recently there is a strong tendency for employment of a digital modulation system or a digital transfer system for transferring video or audio data at a high speed in the fields of mobile communications and digital broadcasting. In the situation described above, now hot attention is concentrating on a multicarrier transfer system based on the OFDM/CDMA modulation system which is a type of multicarrier transfer system. The reasons why this system is attracting hot attention is that, there are advantages such that the capability of this system for suppression of the frequency-selective fading is excellent, the frequency diversity effect is obtained when this system is used together with the technology for error correction coding, it is possible to set a fine frequency interval between subcarriers in this system, and this system allows reduction in the effects due to inter-code interference because it is possible to set a guard interval between symbols in this system.
The OFDM/CDMA modulation system is a technology for subjecting a spectrum-spread signal to OFDM modulation. In this system a frequency interval is set in such a manner that the carriers will be orthogonal to each other within a symbol zone. Further, data transfer is executed by changing the amplitude and the phase of each carrier.
FIG. 6 shows an example of an outgoing signal in the OFDM/CDMA modulation system. It is assumed in this figure that there are eight subcarriers on the frequency axis, and outgoing signals transmitted to two different users are multiplexed and transmitted simultaneously. In FIG. 6, transmitted data arrays D1m and D2m in the transmitter are subjected to spread modulation with a spread code C1n (n is an integer) and spread code C2n (n is an integer) respectively. In other words, the subcarriers are subjected to spread modulation with spread code C11, C12, . . . , C18 and spread code C21, C22, . . . , C28 respectively. The multiplexed signals are then transmitted.
When a signal having been subjected to spread modulation and multiplexing by subcarrier unit passes through a frequency-selective transfer path in which a delayed wave is present, then subcarrier signals corresponding to spread chips are received in a state in which each of the subcarriers has a different amplitude and a different phase as shown in FIG. 7, for example. More specifically, S/N ratio of a spread chip that has been affected by the frequency-selective fading drops. The amplitude and the phase of the signal change in association with such a drop in the S/N ratio. In this case, a type of diversity effect is obtained under influence by the frequency-selective fading in each subcarrier.
In the multicarrier transfer system based on the conventional technology as described above, the spread code C2n, which is originally orthogonal code, is affected by the frequency-selective fading in the same way as the spread code C1n. Accordingly, the spread codes C1n and C2n having an amplitude and a phase which are different from the original ones are received. As a result, the orthogonality between the spread code C1n and the spread code C2n is lost, which causes an interference between the code as well as a degradation in the performance.
In the multicarrier transfer system based on the conventional technology, to prevent the influence of the frequency-selective fading, namely to keep constant the amplitude characteristics and phase characteristics of each chip, a method of, for instance, reducing a number of spread chips (namely a number of subcarriers) is available. However, in the method a spread gain is suppressed, and further it is required to suppress a number of multiplexed carriers when the carriers are subjected to CDMA multiplexing.